1. Field of the Invention
The present invention relates generally to cable deployment systems for laying cables on the sea floor in deep water and a method of using the cable deployment system for laying cables on the sea floor.
2. Description of the Related Art
Offshore oil production has only moved into the very deep waters in recent years, and all known seismic surveys to date in deep water have been done with seismic cables towed behind a vessel of opportunity. New seismic techniques are presenting the need to lay relatively short cables having a total length of only 3 to 5 times the water depth. These new seismic techniques require permanently installing seismic arrays on the sea floor to monitor the depletion of deep water hydrocarbon reservoirs.
The prior art of laying cables on the sea floor in deep water has concentrated primarily on long lines such as intercontinental telecommunications lines. All cables known to have been laid on the sea floor in deep water are long lines and/or are suitable for fabrication with a strength member, or have in some way compromised the need for cost effective, highly reliable installation.
Conventional cable laying vessels are not well suited for handling short cable lengths. Furthermore, since laying short cables does not economically justify employment of a purpose built cable lay vessel, there is a need for a cable laying technique that is compatible with vessels of opportunity.
The method by which short cables in deep water are connected to a surface facility also differs significantly from prior art in that a vertical xe2x80x9criserxe2x80x9d section must be installed from the sea floor to the surface. Seismic cables are highly sensitive cables comprising jacketed electrical conductors and hydrophones or geophones. The requirements of seismic cables are such that the design of the riser section of the cable must be significantly different from the part that lays on the sea floor, although it is necessary to maintain electrical continuity of multiple internal conductors. It is also desirable to minimize the number of electrical connections.
Seismic cable cannot be subjected to tensile forces or compressive loads, which would damages the geophones or break the fine electrical conductors in the cable. Laying seismic cable by hanging it from the deck of a vessel to the sea floor would result in high load forces in the cable due to the weight of suspended cable, vessel heave, water currents, and other perturbations. Lashing the cable to a strength member, such as a wire rope, is a hazardous activity for deck crews, and it extends the vessel time required to lay the cable. Such a requirement also adds expense for the strength member, hold back equipment and lashings. Long lengths of suspended electrical cable are prone to conductor damage due to tension in the cable. Relatively high tensile loads are encountered when cables are deployed to the seafloor from surface vessels in deep water. Strength members are commonly used to take the tension and thereby protect the wires. Strength members must be either inside or outside of the cable. An external strength member is not suitable for use on a seismic cable because it would isolate the geophones and hydrophones from the vibrations they are meant to detect. An internal strength member is not suitable because the device which supports the weight of the suspended cable, be it a winch drum or a tensioning device would impart high compression loads to the geophones and hydrophones. It is desirable to have a means to deploy such cables in deep water in a cost efficient manner without imparting high loads to the electrical conductors or sensors.
A permanently installed riser requires armor to protect it from abrasion, marine life, and other hazards common to the marine environment. It is desirable to make additional use of the armor to provide support for the weight of the cable as it is deployed, as well as the deployment system.
The strength of an electrical signal from a geophone or hydrophone, which is required to drive several kilometers of small gauge wire, is very low. When a long length of this wire is moved through the earth""s magnetic field, as the riser is moved by wave action, the induced electrical noise can result in an undesirable signal-to-noise ratio. It is desirable to provide electromagnetic shielding for the riser section without interfering with the functions of the geophones and hydrophones in the sea floor segment of the cable.
The reliability of electrical connectors is low in comparison to continuous wires. It is desirable to provide a system whereby electrical continuity can be maintained in a combined seismic cable and riser assembly without using connectors.
Ocean currents, which may flow in different directions at various depths in a single water column, make it difficult to control precise cable positioning. Prior art methods for laying subsea cables rely solely on positioning of the surface vessel to determine the position in which the cable lays on the seafloor. It is desirable to have the ability to accurately lay the cable in a predetermined pattern on the seafloor.
The present invention is suitable for laying cable segments on the sea floor in deep water for various purposes, and in particular offers advantages for laying seismic cable comprised of jacketed electrical conductors and hydrophones or geophones.
The cable deployment system of the present invention provides for controlling the attitude of the submersible cable-laying device as the cable is deployed, and for maintaining adequate tension in the cable to deploy it in an orderly manner without inducing excessive forces in the cable. It also includes provisions for determining and controlling the final cable position with greater accuracy than could be achieved if the cable were deployed from the surface.
The cable deployment system provides for a riser to be an integral section of the cable and for all electrical connections in the cable to be factory-made without field splices or connectors.
Conductors in the riser section are isolated from electrical noise, which could be induced as waves and water currents move the riser through the earth""s magnetic field. The present invention also provides for supporting the weight of the riser and for protecting the riser cable against abrasion, shark bites, and other marine hazards.
The present invention also includes provisions for laying cable in specific patterns on the sea floor with high accuracy, including 90-degree turns, regardless of current conditions in the water above the deployment location.
The method of using the cable deployment system comprises lowering the drum of cable to a location just above the sea floor. The drum of cable is deployed by a submersible cable laying system. The riser section of the cable is used as the lowering line, and the drum can be secured to the sea floor to serve as an anchor base for the riser section.